The present invention refers to a geophone and a method for the study of elastic wave phenomena in the ground.
With the aim of studying the lower layers of the terrestrial surface a seismic signal is usually generated and transmitted in the Earth, from one or more sources adjacent to or placed on the terrestrial surface, and have a geophone, i.e. a mechanical-electrical transducer of elastic waves, receive the consequent energy being reflected and/or refracted from the inside of the Earth.
In view of this kind of studies, a certain number of geophones is placed over the ground according to known schemes, to acquire and store all the various information collected through them, usually through an apparatus called seismograph, so as to obtain an indication about the layers underlying the terrestrial ground.
Geophones for the detection of seismic signals generally comprise electromagnetic devices comprising at least a coil, representing the electric circuit, and at least a magnet interacting with the coil. One of these two elements, i.e. either the coil or the magnet, is rigidly connected with the geophone structure, whereas the other one is flexibly suspended to the structure itself
In response to the Earth movement generated by the seismic signal transmitted into it for study purposes, all fixed elements of the geophone move integrally with said structure; also the suspended element will tend to move under the effect of the inertial force, but with a delay in respect to the non suspended element, thus generating a relative motion between the two, i.e. between the magnet and the coil.
This relative motion will induce an electric current to the coil, said current forming the geophone output signal.
Geophones according to the type mentioned above are used to detect various kinds of seismic signals generated during exploration of terrestrial layers.
The most common signal is represented by the so-called compression waves, or lengthwise waves, or type P waves, with propagation occurring through alternate rarefactions and compressions in the material; thus, the motion of the infinite planes into which the rocky body can be ideally broken up, occurs to and thro (backwards and forwards) in the wave propagation direction.
The second type of waves is represented by the shear waves or S waves, where the motion of the rock particles is perpendicular to the direction of the wave propagation.
Shear waves whose particles motion is oriented perpendicularly to the propagation direction and on a horizontal plane are called horizontal shear waves or SH waves, whereas shear waves whose particles motion is oriented perpendicularly to the propagation direction and on a vertical plane are called vertical shear waves or SV waves.
Compression waves P are the signals most commonly used to explore the underground and may be generated in different ways, such as the blasting of an explosive, the falling of weights or the use of apparatus generating elastic pulses or trains of elastic pulses in the ground during a procedure called energization. In the course of the data acquisition campaigns it is also normal practice for the detection operator to strike the ground with special hammers to generate compression waves P.
At present several types of geophones are available on the market, namely:
horizontal geophones to study S waves, provided with two output terminals;
vertical geophones to study P waves, also with two output terminals;
biphone geophones to study for example surface waves with elliptic polarization R, having four output terminals;
three-axis geophones to study P, SH and SV waves, with six output terminals.
From U.S. Pat. No. 4,813,029 is also known an apparatus capable of detecting both the compression waves P and the horizontal shear waves SH, as well as an exploring method using such an apparatus.
The apparatus described in the U.S. Pat. No. 4,813,029 has a first and second geophones assembled on a common carrying plate, where the axis of the geophones are inclined to opposite directions at an angle of about 45xc2x0 from the vertical.
In the preferred embodiment of this apparatus, the axis of the geophones are located on the same vertical plane, which is substantially perpendicular to a line extending from the seismic source to the apparatus, or on parallel vertical planes.
Each geophone of said apparatus detects both the compression waves P and the shear waves SH transmitted to the carrying plate, and each geophone generates an output signal proportional to the seismic energy detected by it.
In a second phase, the two single output signals from the two geophones are either summed up to give a compression wave track and/or subtracted to give a shear wave track.
Thus, according to the present daily practice, the man skilled in the art is never able to obtain a pure shear waves track, i.e. not contaminated by compression waves, on the output of the geophones, in spite of the number of geophones available on the ground according to optimized schemes.
Moreover, in the best of instances three-axis geophones have to be used, and therefore the apparatus apt to store the output signals from the geophones, i.e. the seismograph, receives detections of waves of the P, SH e SV type in an alternate way, so that the seismograph channels are all engaged after the execution of a relatively limited number of detections. According to the present state of art, another drawback associated with the use of the geophones already known is that of requiring an energization first in one direction and then in the opposite direction (i.e. near the two opposite sides of the device), in order to obtain waves of equal intensity but of opposite direction, and to carry out the waves either summed up or subtracted according to the kind of wave being required, as previously described.
This procedure not only increases detection times, but does not warrant an absolute equality of the signal generated by the source, as in most instances it is a manual energization (i.e. obtained with the hammers mentioned above), whose intensity and trend cannot be reproduced exactly with time.
Another drawback according to the prior art is that the output signal has to be sent to proper signal process circuits, thus extending the time required for data processing.
Also, another drawback is that the signals from both the terminals of a geophone have to be stored, thus occupying memory space in the seismograph.
The present invention has the purpose of solving the above drawbacks and provide geophone and a method for the study of elastic wave phenomena in the ground which are improved and more efficient with respect to the solutions already known and with an increased sensitivity to horizontally polarized shear waves.
Within this frame, it is a first aim of the present invention to provide a geophone and/or a method for the study of elastic wave phenomena in the ground, having an increased sensitivity to horizontally polarized shear waves SH, which allows acquisition of data that are less affected by the presence of P waves with respect to the known techniques, and which reduces the data acquisition operations.
A second aim of the present invention is to provide a geophone and/or a method for the study of elastic wave phenomena in the ground, which allows to reduce the time required for the data acquisition campaign, requesting only one energization for each measurement instead of two as presently needed according to the known state of art.
A third aim of the present invention is to provide a geophone and/or a method for the study of elastic wave phenomena in the ground, which allows to reduce the time required to process the output signal, assigning to the special type of assembly of the transducers the task of enhancement of a component along one direction and the attenuation of another component to a different direction orthogonal to the first one.
A fourth aim of the present invention is to optimize the use of the memory of a seismograph associated with the geophone according to the invention, for, on one hand, avoiding a storage of signals from both the terminals of the apparatus and, on the other hand, allowing to use all the memory of the seismograph exclusively for the horizontally polarized shear waves SH.
In order to reach the above aims, the object of the present invention are a geophone and a method for the study of elastic wave phenomena in the ground, incorporating the features of the annexed claims, which form an integral part of this description.